The present invention generally relates to methods and systems for providing cardiac pacing therapy. More particularly, the invention concerns methods and implantable stimulation devices to detect a patient""s position and activity status (e.g., going upstairs or downstairs, sitting or standing up from a supine position, etc.) using an accelerometer-based position sensor.
The cardiovascular system typically adjusts promptly and accurately to changes in a person""s position and activity status. For instance, when a person walks or runs upstairs, the cardiovascular system increases the heart rate to compensate for the additional work being performed by the muscles. However, when a person walks downstairs, the cardiovascular system does not typically need to increase the heart rate (at least not nearly as much as when the person is going up the stairs). As another example, when an individual changes from a horizontal or supine position to a sitting or standing position, the cardiovascular system makes frequent and rapid adjustments to the heart rate and blood pressure to ensure sufficient blood flow to the brain.
When such adjustments are not accomplished, orthostatic hypotension can occur. Orthostasis means upright posture, and hypotension means low blood pressure. Thus, orthostatic hypotension describes the effects caused by low blood pressure when changing from a lying to an upright position, or perhaps from running upstairs. Orthostatic hypotension is defined as a decrease of at least 20 mm Hg in systolic blood pressure when an individual moves from the supine to upright position. The symptoms of orthostatic hypotension include dizziness, faintness, or lightheadedness that appear when standing. Other symptoms that often accompany orthostatic hypotension include chest pain, trouble holding urine, impotence, and dry skin from loss of sweating. Some patients with severe orthostatic hypotension are severely incapacitated.
For mobile and active patients with ineffective cardiovascular systems, cardiac stimulation devices are often used to provide pacing therapy that helps the cardiovascular systems meet the patents"" demands. Unfortunately, conventional devices often cannot differentiate among various patient activities based simply on the patient""s muscle activity. For instance, conventional cardiac stimulation may have difficulty discerning whether a patient is walking upstairs or downstairs, or is sitting or standing up following a prolonged period of rest in the supine position. The cardiac requirements for these various activities are significantly different and hence need to be accurately detected.
Ideally, the cardiac stimulation device would detect a patient""s status. Such conditions might give rise to a situation where therapy is desired, such as treating for orthostatic hypotension with an increased pacing rate when the user stands up quickly.
Accordingly, there is a need for improved detection techniques for accurately detecting a patient""s position and activity status so that appropriate pacing therapy can be selected and timely administered.
An implantable cardiac stimulation device is programmed to administer pacing therapy in response to changes in a patient""s position as detected by a 3D accelerometer-based position sensor. Such changes include climbing vertically upward or downward (e.g., walking upstairs/downstairs, rock/mountain climbing, etc.) and moving from a supine position to an upright position. Depending upon the position/activity change, a suitable pacing therapy (or omission of therapy) is applied, such as increasing the cardiac pacing rate when the patent walks upstairs, or sits up, to counteract any effects of orthostatic hypotension.
In the described implementation, the cardiac stimulation device is equipped with an accelerometer-based sensor to sense a patient""s movement and produce a vertical acceleration component indicative of a patient""s acceleration in a vertical direction. A vertical velocity component is then integrated from the vertical acceleration component. The device may be further equipped with a magnetic field sensor to sense the earth""s magnetic field as a way to obtain a true vertical orientation, against which the accelerometer-based sensor can be calibrated. The device may further include a minute ventilation sensor to sense minute ventilation of the patient.
The cardiac stimulation device includes a processor coupled to the various sensors. The processor is programmed to determine a patient""s position and activity status and whether to administer cardiac pacing therapy to the patient based on the vertical velocity and minute ventilation data. For instance, the processor might direct a pacing generator to increase a pacing rate in the event that the vertical velocity indicates an upward velocity and the minute ventilation shows an increase in breathing activity above a threshold.